Storm-induced hydrodynamic changes and seabed erosion in the littoral area of Yellow River Delta: A model-guided mechanism study

The Gorumara National Park (GNP) is an important conservation area located in the northern region of West Bengal State, India, as it provides habitat for three megaherbivores: Indian One-horned rhinoceros (Rhinoceros unicornis), Asian elephants (Elephas maximus) and Gaurs (Bos gaurus). It harbours one of the last population of the one-horned rhino. In the present study, landscape change and configuration were investigated by comparing three Landsat images, from 1998, 2008 and 2018. The images were classified into six different landcover classes following standard methodology. The present study also involves evaluation of landscape and anthropogenic predictors influence on the megaherbivores of GNP, followed by future landcover simulation for the year 2028. The result shows a significant decrease in the grassland cover from 18.87 km2 to 8.27 km2 from 1998 to 2018, whereas the woodland cover has increased from 50.14 km2 to 62.09 km2 between 1998 and 2018. The landscape configuration indices such as Number of Patches (NP), Patch Density (PD), Interspersion and Juxtaposition (IJI), Aggregation Index (AI) and Mean Shape Index (SHAPE AM) indicated that the landscapes has lost complexity in the spatial placement of patches of different Land Use and Land Cover (LULC) classes. Also, the landscape over the three decades has become uniform in terms of diversity of patches, because of earlier plantation activities by the forest managers. Result also indicated that grassland, along with its class metrics are the top predictors contributing 43.6% in explaining the spatial distribution of megaherbivores in GNP. Results from the simulated landcover of 2028 suggest a possible decline in overall grassland by 6.23% and a subsequent upsurge in woodland by 6.09% from 2018. The present result will be useful in guiding the forest management in developing habitat improvement strategies for the long- term viability of megaherbivore populations of rhino, gaur and elephant in the GNP.

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Changing landscape configuration demands ecological planning: Retrospect and prospect for megaherbivores of North Bengal.

We modified the sediment incipient motion in a numerical model and evaluated the impact of this modification using a study case of the coastal area around Weihai, China. The modified and unmodified versions of the model were validated by comparing simulated and observed data of currents, waves, and suspended sediment concentrations (SSC) measured from July 25th to July 26th, 2006. A fitted Shields diagram was introduced into the sediment model so that the critical erosional shear stress could vary with time. Thus, the simulated SSC patterns were improved to more closely reflect the observed values, so that the relative error of the variation range decreased by up to 34.5% and the relative error of simulated temporally averaged SSC decreased by up to 36%. In the modified model, the critical shear stress values of the simulated silt with a diameter of 0.035 mm and mud with a diameter of 0.004 mm varied from 0.05 to 0.13 N/m2, and from 0.05 to 0.14 N/m2, respectively, instead of remaining constant in the unmodified model. Besides, a method of applying spatially varying fractions of the mixed grain size sediment improved the simulated SSC distribution to fit better to the remote sensing map and reproduced the zonal area with high SSC between Heini Bay and the erosion groove in the modified model. The Relative Mean Absolute Error was reduced by between 6% and 79%, depending on the regional attributes when we used the modified method to simulate incipient sediment motion. But the modification achieved the higher accuracy in this study at a cost of computation speed decreasing by 1.52%.

A method for simulating sediment incipient motion varying with time and space in an ocean model (FVCOM): development and validation

Most of the existing results were related to the impact of land storms on sediment, and few explained the impact of storms on sediment movement and its formation law from the mechanism. At present, the research on the impact of storm on sediment suspension on the North Bank of Longkou City in the south of Bohai Sea was not clear. Therefore, this study aimed to explore the formation and mechanism of storm on suspended sediment from the perspective of hydrodynamic characteristics and sediment distribution by monitoring the impact of storm on sediment suspension in the south of Bohai Sea. A storm with wind speeds of 4.5–13.5 m/s occurred in the coastal area of the north side of Longkou City located in Southern Bohai Sea in April, 2015. Suspended sediment samples were obtained using automatic samplers, and the results showed that the suspended sediment content was 7.8 mg/L under normal weather conditions, which reached 121.2 mg/L at the highest during the storm event. Waves and tides were synchronously observed by acoustic Doppler current meter. Wave height was more closely correlated with wind speed when the wind veered to north, and the maximum wave height was 243 cm. The reciprocating motion of the current at the sampling site was strong, with the maximum current speed of 43.8 cm/s and a water depth of 10 m. The results of laser analysis showed that the bottom sediments were composed of 92% sand and 8% silt, with a medium diameter of 0.102 mm. From the experimental observation and result analysis, it was known that the strong dynamic process during storm surge led to the movement of sediment surface and the resuspension of sediment. 

Increase in Suspended Sediment Contents by a Storm Surge in Southern Bohai Sea, China

Abstract. The inner shelf and coastal region of the Yellow Sea along the Korean
Peninsula are frequently impacted by typhoons. The Mokpo coastal region in
South Korea was significantly affected by Typhoon Soulik in 2018, the
deadliest typhoon strike to the southwestern coast since Typhoon Maemi in 2003.
Typhoon Soulik overran the region, causing extensive damage to the coast,
shoreline, vegetation, and coastal geomorphology. Therefore, it is important to investigate its impact on the coastal ecology, landform,
erosion/accretion, suspended-sediment concentration (SSC), and associated
coastal changes along the Mokpo region. In this study, the net shoreline movement (NSM), normalized difference
vegetation index (NDVI), fractional vegetation coverage (FVC), coastal-landform change model, normalized difference suspended-sediment index
(NDSSI), and SSC–reflectance relation have been used to analyze the coastal
morphodynamics over the typhoon periods. We used pre- and post-typhoon
Sentinel-2 MultiSpectral Instrument (MSI) images for mapping and monitoring the typhoon effect and
recovery status of the Mokpo coast through short- and medium-term coastal-change analysis. The findings highlighted the significant impacts of
typhoons on coastal dynamics, wetland vegetation, and sediment resuspension
along the Mokpo coast. It has been observed that typhoon-induced SSC
influences shoreline and coastal morphology. The outcome of this research
may provide databases to manage coastal environments and a long-term plan to restore valuable coastal habitats. In addition, the findings may be useful for post-typhoon emergency response, coastal planners, and administrators involved in the long-term development of human life.


Assessing Typhoon Soulik-induced morphodynamics over the Mokpo coastal region in South Korea based on a geospatial approach

The suspension and hydrodynamic characteristics of the Yellow River Delta during storms were analyzed based on suspended samples obtained using automatic samplers during a storm event in the Yellow River Delta. Synchronous data for winds, waves, and tides were also collected from a nearby station. The results show that under wind speeds of 5–15 m/s and wave heights of 50–150 cm, the suspended content reached 5.7–49.6 kg/m3, which is 10–100 times higher than that under normal weather conditions. The medium diameter of suspended particles was 1.2–2.1 μm (8.9–9.7 Φ), which was approximately 1–2 Φ finer than that under normal weather conditions. During the early stages of the measurements, the sea level had risen by 50 cm owing to the storm, which was in addition to the tidal sea level change. We suggest that during the storms, the waves strengthened and the storm-induced sea level change, which was combined with tidal currents moving in the same direction, produced high-speed currents. This overcame the cohesive forces among the fine sediment particles and suspended a large amount of sediment. As a result, the suspended content increased markedly and the suspended particle size became finer. This explains the intense siltation and erosion of the Yellow River Delta during storms.

Jianqiang Liu

Papers

Sediment suspension and the dynamic mechanism during storms in the Yellow River Delta